Tunable Lasers

Before reading this you may find the following tutorials useful:
Wavelength Division Multiplexing (WDM), Laser Basics, Distributed Feedback (DFB) Lasers, Submarine Systems

Imagine that we live in a world where utensils are extremely rare — a fork costs as much as your yearly salary, and a spoon is similarly expensive. (Hey, it could happen.) You have one fork and one spoon at home, and you rely on them for all of your meals. You use the spoon in the morning to eat your cereal, you use the fork at lunch to eat some pasta, use it again at dinner time for your chicken and rice dish. Without a fork and a spoon, your life would be miserable. Despite your years of protest and campaigns, the price of cutlery is not going to drop. What if you lost one of them? It's hard to contemplate how you could continue to live in a world where you didn't own a fork. To cater for this possibility, you have spent two years' worth of your hard earned cash to have a backup fork and a backup spoon in case this nightmare situation ever comes true.

In a wavelength-division multiplexed (WDM) network carrying 128 wavelengths of information, you have 128 different lasers giving out these wavelengths of light. Each laser is designed differently in order to give you the exact wavelength you need. Because you want to plan for the event that a laser breaks down, you need to keep spare lasers. Even though the lasers are expensive, you have to be able to replace one at a moment's notice so that you don't lose any of the capacity that you have invested so much money in. So you keep in stock 128 spare lasers, or maybe even 256 if you want to be prepared for double failures.

But wait a minute, have you ever heard of a spork? Well, it is a spoon and a fork combined into one. What a great idea! If only you had known about that, you could have saved yourself a year's salary. Then you could have used it as a replacement for either your fork or your spoon, depending on which you lost. Similarly, what if you could have a multifunctional laser for your optical network that could be adapted to replace one of a number of lasers out of your total 128 wavelengths? Think of the money that could be saved, as well as the storage space for your spares. What you need is a “tunable laser,” the optical networking spork.

Tunable lasers are still a relatively young technology, but as the number of wavelengths in networks increases so will their importance. Each different wavelength in an optical network will be separated by a multiple of 0.8 nonometer (sometimes referred to as 100GHz spacing, which is the frequency separation, or as the “ITU-Grid” — named after the standards body that set the figure). Current commercial products can cover maybe four of these wavelengths at a time — twice as good as a spork. While not the ideal solution, this still cuts your required number of spare lasers down from 128 to 32. A simple adjustment to the laser can set it to emit at any of these four specified wavelengths. More advanced solutions hope to be able to cover larger number of wavelengths, and should cut the cost of spares even further.



The devices themselves are still semiconductor-based lasers that operate on similar principles to the basic non-tunable versions. Most designs incorporate some form of grating like those in a distributed feedback laser, but much more complex in their implementation. These special gratings can be altered in order to change which wavelengths they reflect in the laser cavity, usually by running electric current through them, thereby altering their refractive index — a key property directly related to reflection properties. The tuning range of such devices can be as high as 40nm, which would cover any of 50 different wavelengths in a 0.8nm wavelength spaced system. Technologies based on vertical cavity surface emitting lasers (VCSELs) incorporate moveable cavity ends that change the length of the cavity and hence the wavelength emitted. Current designs of tunable VCSELs have similar tuning ranges.

Key Points

Further Reading

Distributed Feedback (DFB) Lasers, Vertical Cavity Surface Emitting Lasers (VCSELs)